This invention relates to a hearing aid of the type having a sound pressure outlet passageway that, in use, is directly coupled to the ear canal and more particularly to a hearing aid with an improved output transducer for suppressing feedback occasioned by the mechanical vibration of the case that houses the transducer.
Feedback problems are present in all sound amplification systems that involve an input microphone and an electroacoustical transducer for generating acoustical sound pressures and where the output transducer in located in the proximate area of the microphone or sound input transducer. With public address or P.A. systems, an acoustic air path exists between the output transducer and microphone, and sound pressures generated by the output transducer can traverse an air path back to the input microphone. In such "open acoustical systems", if the total gain exceeds about 1:1 at the interface of the vibrating member of the input microphone, sustained regenerative oscillations are set up. The amount of overall signal amplification usable in such systems is normally limited so that which doesn't produce the sustained oscillations caused by the feedback of the transducer output ot the input microphone.
In hearing aid systems, feedback problems are aggravated by the close proximate locations of the microphone and output transducer and by the further fact that it is not uncommon for an acoustical gain of 1000:1 to be required in order to compensate for the hearing impairment of the hearing aid user. With this magnitude of amplification, the transducer output pressure must be channeled directly into the user's ear canal through the use of a properly fitting ear mold so as to avoid establishment of a return air path to the microphone. In this type of "closed acoustical system", however, feedback is still a major problem because mechanical vibrations from the mechanical parts of the transducer are imparted to the case that houses the output transducer. Such mechanical case vibrations generate sound pressures in the surrounding air which find an air path back to the microphone, and all despite the basically closed nature of the acoustical system.
While the problems of case vibration and their resultant effect in producing feedback are known, the efforts to solving the problems have been primarily directed to providing vibration dampening structures in the casing area which surrounds the output transducer housing and, at the expense of added weight, to strengthening the housing and supporting structures for the transducer components so as to add a vibration attenuating mass factor to the supporting structure.
It has been known, since long prior to the advent of the modern hearing aid systems, that vibrations imparted to the base mount of a loud speaker can be minimized by using a pair of diaphragms that are mounted in a face-to-face relation, if the diaphragms are driven in opposite directions by symetrically arranged drivers (see British Pat. No. 241,343). However, when such a speaker arrangement is housed in a surrounding case, the air chambers at the back sides of the diaphragms act as resonance chambers and the case vibrates for reasons of an air coupling between the diaphragms and the casing walls. This can only be partly suppressed through the extensive use of dampening materials, and as far as is known, this type construction has not enjoyed commercial acceptance.
The ultimate effects of feedback in hearing aids are several. For one, the feedback problems in hearing aids have always limited the amount of amplification gain that could be attained without interference from the sustained oscillation that develop from feedback. Consequently, potential users who need higher amplifications than those permissible because of the sustained oscillations have been deprived of the use of hearing aids. Secondly, the feedback problems with hearing aids have actually caused damage to the hearing of some users. For example, it is known that through periodic exposure to the sustained oscillations caused by feedback, some users have developed an inability to detect the oscillations and hence, have lost a sense of perception of the frequencies involved. Thirdly, many hearing aid manufacturers purposely design their circuitry to provide a poor response in the higher frequency ranges in order to avoid some of the feedback problems. As such, the users are denied the hearing of a full frequency spectrum.